Wireless communications system and method

ABSTRACT

A wireless communications system is described which comprises a base station, a relay device, and a terminal device. The terminal device is operable to transmit a message to the base station via the relay device. The relay device is operable to add a relay header to the message received from the terminal device, the relay header comprising control information for controlling the transmission of subsequent messages from the terminal device to the relay device. The relay device is also operable to relay the message having the relay header added to the base station. By adding such control information to messages being relayed from the terminal device to the base station on the uplink, subsequent downlink communications from the base station to the terminal device can include the control information generated at the relay device (or transmission parameters derived from the control information).

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from GB Application No. 1114344.3 filedon Aug. 19, 2011, entitled “WIRELESS COMMUNICATIONS SYSTEM AND METHOD,”commonly assigned with the present invention and incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to a wireless communications system andmethod. Further aspects of the present invention relate to a basestation, a relay device, a terminal device and a computer program.

BACKGROUND OF THE INVENTION

Machine Type Communication (MTC) is being discussed in various wirelesscommunication standards bodies as a new trend of wireless technology ofcommunication network applications which typically do not require humaninteraction.

A broad definition of MTC is an automated communication network to andfrom machines. One major category of MTC devices are expected to havethe characteristics of very low power consumption, very small datatransmissions and a very large number of terminals. An example MTCapplication that fits within this category may for example be energyconsumption monitoring of home appliances for smart grid systems.

Uplink-only relaying is a network topology which may be used to addressthe issue of transmit power limitation in low-cost Machine TypeCommunication (MTC) devices when deployed, for instance, in macrocellular Long-Term-Evolution (LTE) networks. In general there issufficient system gain on the downlink (base station to terminal device)to support MTC devices (or User Equipment UE) (MTC-UE) at the cell edgeof the macrocell of the eNodeB (eNB) but with the low output power ofthe MTC devices the uplink (terminal device to base station) system gainis significantly reduced compared with the downlink. The use of a singlehop uplink-only relay node (MTC-RN) can be used to address this issueand close the link budget for MTC-UE. A single hop may be assumedprovided that the MTC-RN can be expected to have similar characteristicsto an LTE UE. In a network where relay nodes (also referred to herein asrelay devices) are utilised to relay uplink data from the terminaldevices to the eNodeB, the eNodeB may be referred to as a donor eNodeB(DeNB).

An uplink only single-hop relay is described in US2008/0285499.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided awireless communications system, comprising:

a base station;

a relay device; and

a terminal device; wherein

the terminal device is operable to transmit a message to the basestation via the relay device; and

the relay device is operable to

add a relay header to the message received from the terminal device, therelay header comprising control information for controlling thetransmission of subsequent messages from the terminal device to therelay device; and

relay the message having the relay header added to the base station.

A problem with uplink-only relaying is that the relay device cannot feedback control information to the terminal device to support efficientfuture transmissions over the air interface between the terminal deviceand the relay device. However, by adding such control information tomessages being relayed from the terminal device to the base station onthe uplink, subsequent downlink communications from the base station tothe terminal device can include the control information generated at therelay device (or transmission parameters derived from the controlinformation).

As such, the base station may be operable to transmit the controlinformation contained in the relay header to the terminal device, andthe terminal device may then be operable to transmit one or moresubsequent messages to the relay device based on the control informationincluded in the relay header.

In one embodiment, processing at the relay device takes place at theMedium Access Control (MAC) layer. In this case, a MAC relay headercontrol element can be used to provide uplink-only relayingfunctionality. The MAC relay header control element may includeACK/NACK, power control and timing advance information for the terminaldevice. The power control and timing advance information can then betransmitted to the terminal device to control the power level and timingof transmissions from the terminal device to the relay device.

In particular, in an embodiment, the relay device is operable

to measure a received signal power of the message received from theterminal device; and

to set power control information in the control information forcontrolling a power level of a subsequent transmission from the terminaldevice to the relay device. The power control information may specify apower offset from the received signal power (the power level at whichthe message was received from the terminal device at the relay device).The terminal device may be responsive to the power control informationto control the transmission power of a subsequent transmission to therelay device.

Similarly, in an embodiment the relay device is operable

to measure a propagation time delay between the terminal device and therelay device of the message transmitted from the terminal device to therelay device;

to determine, based on the propagation time delay, a required timingadvance offset for a subsequent transmission from the terminal device tothe relay device so that a receive time of the subsequent transmissionsat the relay device is synchronised with a receive time of transmissionsfrom other terminal devices; and

to specify said timing advance offset in the control information.

The terminal device may be responsive to the timing advance informationto control the timing of a subsequent transmission from the terminaldevice to the relay device.

The relay device may be operable to transmit the relay header to thebase station in relation to an expected message from the terminal deviceeven if the expected message was not correctly received at the relaydevice.

The message from the terminal device can be considered to be expected ifuplink resources have been allocated to the terminal device by the basestation.

In an embodiment, the relay device is operable

to combine a plurality of messages received from one or more terminaldevices into a relay message, the relay header being added to the relaymessage and having a respective relay control element for each of theplurality of messages received at the relay device, each relay controlelement comprising the control information for controlling thetransmission of subsequent messages from the terminal device originatingthe message to which the relay control element relates; and

to transmit the relay message to the base station.

The relay header may comprise an identification of the terminal devicewhich transmitted the message. A relay control element for an expectedmessage from a terminal device may be included in the relay header evenif the expected message is not correctly received at the relay device.In this case, the relay control element may comprise an indication ofwhether the associated message is present in the relay message. The basestation may be responsive to the absence of an expected message in therelay message to transmit a negative acknowledgement message to theterminal device.

In an embodiment, the relay header comprises further control informationfor controlling the transmission of data between the relay device andthe base station.

In an example ordering for the relay message, the relay control elementsin the relay header are in the same order as the messages to which theycorrespond. The relay control elements in the relay header may bepositioned after the further control information. When a plurality ofmessages from the same terminal device are present in the relay message,they may be positioned within the relay message in the order in whichthey were received at the relay device.

The relay device is operable to process and relay the message receivedfrom the terminal device at a Medium Access Control (MAC) layer in aprotocol stack. It has been recognised that it is possible to provideadequate relaying functionality without resorting to higher layers inthe protocol stack.

The messages received at the relay device from the terminal devices maybe MAC Protocol Data Units. The relay message may be a MAC Protocol DataUnit. In this case, the base station may be operable

to process the MAC Protocol Data Unit of the relay message at the MAClayer in a protocol stack to extract a MAC Protocol Data Unit of themessage from the terminal device;

to process the MAC Protocol Data Unit of the message from the terminaldevice at the MAC layer; and

to pass the processed MAC Protocol Data Unit of the message from theterminal device to a higher level in the protocol stack.

According to another aspect of the present invention, there is provideda method of wirelessly communicating data from a terminal device to abase station via a relay device, comprising:

transmitting a message from the terminal device to the base station viathe relay device;

at the relay device, adding a relay header to the message received fromthe terminal device, the relay header comprising control information forcontrolling the transmission of subsequent messages from the terminaldevice to the relay device; and

relaying the message having the relay header added to the base station.

According to another aspect of the present invention, there is provideda base station for receiving wireless communications from a terminaldevice via a relay device, the base station comprising:

a receiver for receiving a message transmitted from the terminal devicevia the relay device; wherein

the message comprises a relay header added by the relay device, therelay header comprising control information for controlling thetransmission of subsequent messages from the terminal device to therelay device.

According to another aspect of the present invention, there is provideda relay device for wirelessly relaying data from a terminal device to abase station, the relay device comprising:

a receiver for receiving from the terminal device a message for relayingto the base station; and

a transmitter for adding a relay header to the message received from theterminal device, the relay header comprising control information forcontrolling the transmission of subsequent messages from the terminaldevice to the relay device and relaying the message having the relayheader added to the base station.

According to another aspect of the present invention, there is provideda terminal device for wirelessly communicating data to a base stationvia a relay device, the terminal device comprising:

a transmitter for transmitting a message to the base station via therelay device; wherein

the relay device is operable to

add a relay header to the message received from the terminal device, therelay header comprising control information for controlling thetransmission of subsequent messages from the terminal device to therelay device; and

relay the message having the relay header added to the base station.

Further aspects and features of the present invention are defined in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments of the present invention will now be described withreference to the accompanying drawings in which like parts have the samedesignated references and in which:

FIGS. 1A and 1B provide a comparison of a symmetric uplink/downlinkmulti-hop signalling scenario and an asymmetric uplink/downlinksignalling scenario for a relay network;

FIG. 2 is a schematic block diagram of a mobile communications networkand mobile communications devices forming a communication system whichoperates in accordance with the 3GPP Long Term Evolution (LTE) standard;

FIG. 3 schematically illustrates an example downlink data and controlchannel structure for use in the network shown in FIG. 2;

FIG. 4 schematically illustrates an example uplink data and controlchannel structure for use in the network shown in FIG. 2;

FIG. 5 schematically illustrates a random access procedure for a mobileterminal to connect to a network;

FIG. 6 schematically illustrates User Equipment (UE) Physical RandomAccess Channel (PRACH) (UE PRACH) power ramping and the interception ofrandom access preambles at a relay device;

FIG. 7 schematically illustrates a technique for using the random accessprocedure to trigger a relay to serve a terminal device;

FIG. 8 schematically illustrates a new MAC control element for relayinga random access request to a base station;

FIG. 9 schematically illustrates a modified random access response (RAR)message;

FIG. 10 schematically illustrates a method for arbitrating betweenmultiple possible serving relays;

FIG. 11 schematically illustrates a method for determining the mostappropriate relay device to serve a terminal device;

FIG. 12 schematically illustrates a MAC control element for indicatingwhich relay device is to serve a terminal device;

FIG. 13 schematically illustrates a protocol stack configuration andsignal flow for a terminal device, relay device and base station fordownlink signals;

FIG. 14 schematically illustrates a protocol stack configuration andsignal flow for a terminal device, relay device and base station foruplink signals;

FIG. 15 schematically illustrates the encapsulation and relay of MACmessages at a relay device;

FIG. 16 schematically illustrates an example format for a MAC relayheader control element;

FIG. 17 schematically illustrates an example MAC relayed PDU in whichall indicated MTC-UE MAC PDUs are present;

FIG. 18 schematically illustrates an example MAC relayed PDU in whichsome of the indicated MTC-UE MAC PDUs are not present;

FIG. 19 schematically illustrates an example procedure for communicatingbetween a terminal device and a base station via a relay device foruplink communications;

FIG. 20 schematically illustrates a first technique for the relay deviceto become aware of uplink resources to use for receiving a message froma terminal device, and for relaying the received message to a basestation;

FIG. 21 schematically illustrates how the relay device behaves in thefirst technique when the message from the terminal device is notreceived;

FIG. 22 schematically illustrates a second technique for the relaydevice to become aware of uplink resources to use for receiving amessage from a terminal device, and for relaying the received message toa base station;

FIG. 23 schematically illustrates a third technique for the relay deviceto become aware of uplink resources to use for receiving a message froma terminal device, and for relaying the received message to a basestation;

FIG. 24 schematically illustrates a fourth technique for the relaydevice to become aware of uplink resources to use for receiving amessage from a terminal device, and for relaying the received message toa base station;

FIG. 25 schematically illustrates an example implementation of therelayed scheduling request (SR);

FIG. 26 schematically illustrates an example implementation of the newMAC control element for indicating to the MTC-RN on which uplinkresources to expect a transmission from the MTC-UE; and

FIG. 27 schematically illustrates a wireless communications systemcomprising a DeNB, an MTC-RN and an MTC-UE.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Referring first to FIGS. 1A and 1B, a comparison of a symmetricuplink/downlink single-hop signalling scenario and an asymmetricuplink/downlink signalling scenario when applied to a relay network isprovided. FIG. 1A illustrates a symmetric uplink/downlink case in whichcommunications are hopped both on a downlink 10 a from a base station 7a to several terminal devices 9 a via relay devices 8 a, and also on anuplink 11 a from the terminal devices 9 a to the base station 7 a viathe relay devices 8 a. FIG. 1B illustrates an asymmetric uplink/downlinkcase in which communications are direct on a downlink 10 b from a basestation 7 b to a terminal device 9 b, but are single-hopped on an uplink11 b from the terminal device 9 b to the base station 7 b via the relaydevices 8 b. The base station 7 b may also transmit control signallingon a downlink 12 b (dashed lines in FIG. 1B) to the relay devices 8 b inorder to control the operation of the relay devices 8 b. Notably thoughthere is no transmission from the relay devices 8 b to the terminaldevices 9 b. It will be appreciated that the asymmetric uplink/downlinkrelay configuration of FIG. 1B will receive a number of benefits.Firstly, it will be understood that hopping permits lower powertransmissions to be used, but increases transmission time. Hopping ofboth the control signal and the data signal (as conducted in FIG. 1A)takes a relatively long time to be conveyed, due to the processing delayon reception/retransmission at an intermediate step in the route. Incontrast, the transmission time is reduced in relation to the controlsignal sent from the base station (BS) to the terminal by transmittingthe control signal directly. While on the face of it this goes againstthe low transmission power goals of a relay type network, in reality thebase station will not be subject to the same transmission powerconstraints as the terminal device and relays within the network.Moreover, this arrangement is particularly advantageous for MTC devicesbecause of the relatively high volume of control signalling versus datasignalling which is typically utilised for this type of device.

Certain embodiments of the present invention will now be described withreference to an implementation which uses a mobile communicationsnetwork operating in accordance with the 3GPP Long Term Evolution (LTE)standard. FIG. 2 is a schematic block diagram of a mobile communicationsnetwork and mobile communications devices forming a communication systemwhich operates in accordance with the 3GPP Long Term Evolution (LTE)standard (Release 8 of the specification). The mobile network includes aplurality of base stations known in the art as enhanced Node-Bs 101(eNBs) each of which includes a transceiver unit 103 enablingcommunication of data to and from a plurality of mobile communicationdevices 105 via a radio interface. Each mobile communication device 105includes a transceiver for communicating data to and from the eNBs and aUSIM which uniquely identifies the mobile communication device.

Each eNB 101 provides a coverage area (i.e. a cell) and communicatesdata to and from mobile communication devices 102 within the coveragearea/cell. Each eNB 101 is connected to a Serving Gateway (S-GW) 104which routes user data to and from the eNBs 101 and supports mobilitywhen mobile communication devices 105 handover between eNBs 101 as isknown in the art.

The mobile network is typically divided into a number of tracking areaseach of which comprise a number of eNBs. Together the tracking areasform a network coverage area providing access to the Public Land MobileNetwork (PLMN) over a geographic area. The S-GW 104 is connected to aPacket Data Network Gateway 106 (P-GW) which is the network entity fromwhich packet data is routed into and routed out of the network. Themobile telecommunication network also includes a Mobility ManagementEntity 107 (MME) connected to the S-GW 104. The MME 107 is responsiblefor authenticating mobile communication devices 105 attempting to accessthe network by retrieving subscriber profile information stored in aHome Subscriber Server 108 (HSS). The MME 107 also tracks the locationof each mobile communication device 105 that has joined the network. TheeNBs grouped together form a radio network part of the PLMN and theinfrastructure equipment of the PLMN, namely the S-GW, MME and P-GW forma core network part of the PLMN.

FIG. 3 schematically illustrates an example downlink data and controlchannel structure for use over the air (Uu) interface between the eNBs101 and communication devices 105 in the LTE based network of FIG. 2.According to the LTE standard, a physical downlink frame is used tocommunicate control signalling and data on the downlink (base station toterminal device). FIG. 3 is a somewhat simplified form of this, forexample a LTE frame usually includes 10 sub-frames but only 6 sub-frames130 have been represented for the downlink frame 120 of FIG. 3. Belowthe representation of the LTE frame 120 in FIG. 3 is an expanded versionof one of the sub-frames 130. In each sub-frame 130, a Physical DownlinkControl Channel (PDCCH) 140 is shown which occupies time and frequencyresources that stretch across the entire frequency band (vertical) andacross 1 to 3 symbols in the time axis (horizontal). In contrast thePhysical Downlink Shared CHannel (PDSCH) 150 is comprised of a pluralityof time and frequency resources which are allocated via the PDCCH. Ineffect, the PDCCH provides the mobile communications devices with theresource allocations and the corresponding addressing information (forexample the radio network temporary identifier—RNTI). A mobilecommunications device can therefore, based on the RNTI, know whichresource allocations it should decode to receive data intended for(addressed to) it. The data may be either data for this mobilecommunications device only or for all mobile communications devices inthe cell. In FIG. 3, two resource blocks 162, 164 are highlighted. Thesecould be allocated to a particular terminal device by controlinformation provided in the PDCCH 140 in association with the RNTI ofthat particular terminal device. The terminal device would then know todecode data transmitted in that frequency/symbol allocation.

In a similar fashion, FIG. 4 schematically illustrates an example uplinkdata and control channel structure for use over the air (Uu) interfacebetween the eNBs 101 and communication devices 105 in the network shownin FIG. 2. As with the downlink, a physical uplink frame 220 is used tocommunicate control signalling and data on the uplink (terminal deviceto base station). Again, as with FIG. 3, FIG. 4 is a somewhat simplifiedform of this. In FIG. 4, the physical uplink frame 220 is divided intosub-frames 230. Below the representation of the LTE frame 220 in FIG. 4is an expanded version of one of the sub-frames 230. In each sub-frame230, a Physical Uplink Control Channel (PUCCH) 240 is shown whichoccupies some time and frequency resources that stretch across theentire time (symbol) band (horizontal) and across a portion of the upperand lower extremities of the frequency band (vertical). In contrast thePhysical Uplink Shared CHannel (PUSCH) 250 is comprised of a pluralityof time and frequency resources which are allocated via the PDCCH (inthe downlink frame). The PDCCH therefore provides the mobilecommunications devices with the resource allocations and thecorresponding addressing information (for example the radio networktemporary identifier—RNTI) for the transmitting as well as receivingcontrol signalling and data. A mobile communications device cantherefore, based on the RNTI, know which resource allocations it shouldtransmit data on. In FIG. 4, two resource blocks 262, 264 arehighlighted. These could be allocated to a particular terminal device bycontrol information provided in the PDCCH 240 in association with theRNTI of that particular terminal device. The terminal device would thenknow to transmit data using that frequency/symbol allocation.

It will be appreciated that in the context of the asymmetric relaytopology described in FIG. 1B, each of the base station, the relaydevice and the terminal device will make use of these uplink anddownlink control and shared channels to communicate information betweenthem. In particular, the base station is able to communicate informationto both the terminal device and the relay device using PDCCH and PDSCH.Typically the base station will utilise the PDCCH to indicate to therecipient device where on the PDSCH to look for data intended for therecipient device. The user device is able to communicate information toeither the base station (if it is in range) on the PUCCH and the PUSCH,and is also able to communicate information to the relay device on thePUCCH and the PUSCH. The relay device is able to communicate informationto the base station on the PUCCH and the PUSCH. In some embodiments therelay device is required to request PUSCH resources for relaying datafrom the terminal device received at the relay device on the PUSCH. Insome embodiments the terminal device and relay device are provided withallocated (time/frequency) resources on the PUCCH which they can utilise(by transmitting on those resources) to request the base station toallocate time/frequency resources on the PUSCH. The PUCCH resourcesallocated to the terminal device and the relay device may bepredetermined. In some embodiments the relay device is aware of thePUCCH resources allocated to the terminal device so that it can receiveand relay requests for uplink resources to the base station. The relaydevice may be aware of the PUCCH resources allocated to the terminaldevice either due to the predetermined nature of the allocation, or byway of a direct notification from the base station. In an alternativeembodiment the terminal device may identify itself in any PUCCHtransmissions, enabling the relay device to monitor the PUCCH and pickout any PUCCH transmissions made by a terminal device for which it isresponsible.

Random Access Procedure

One mechanism for identifying whether a terminal device needs to beserved by a relay device, and the appropriate relay device for theterminal device to use, would be for the relay device to observe therandom access procedure. The random access procedure is the method bywhich a terminal device obtains access to the base station. The randomaccess procedure is conducted in relation to several events, includinginitial access to the network, connection re-establishment, handoverbetween radio cells, downlink data arrival and uplink data arrival. Aschematic illustration of the random access procedure of an LTE networkis shown in FIG. 5.

In FIG. 5, a signal flow between a terminal device (UE) and a basestation (BS) is shown. Four stages are illustrated. A first stage, A, isthe transmission of one or more random access preambles from theterminal device to the base station. The terminal device randomlyselects a random access preamble (code sequence) from a group ofpredetermined random access preambles. The preamble is transmitted on anext available Physical Random Access Channel (PRACH) resource which isavailable to the terminal device. The position in time and frequencyresource of the transmitted random access preamble implicitly provides atemporary identifier (RA-RNTI) for the transmitting terminal device.

If the random access preamble is received at the base station, at astage B a random access response is transmitted from the base station tothe terminal device. The random access response is transmitted on thePDSCH, with the terminal device being informed by the PDCCH of thedownlink resources on which the random access response is to be carried.In particular, the PDCCH specifies the temporary identifier derived fromthe time and frequency resources utilised to transmit the random accesspreamble, and also indicates a resource block assignment (time andfrequency resources) on the PDSCH which will carry the random accessresponse. The terminal device recognises that the random access responseis intended for it by way of the temporary identifier, and looks at theassigned resource block in the PDSCH. The MAC header in the RandomAccess Response on the PDSCH includes a random access preambleidentifier which identifies the random access preamble received at thebase station at the stage A, a further temporary identifier (C-RNTI) foridentifying the terminal device, a grant of uplink resources on thePUSCH, and a timing advance command for adjusting transmission times atthe terminal device in dependence on the distance between the terminaldevice and the base station.

In response to the random access response, at a stage C the terminaldevice transmits a message 3 ((Radio Resource Control (RRC) ConnectionRequest Message) to the base station. The message 3 is transmitted onthe PUSCH resources allocated by the random access response.

In response to the message 3, at a stage D the base station sendscertain information to the terminal device for contention resolutionpurposes. This information is transmitted on the PDSCH (again onresources allocated by the PDSCH). This information includes the furthertemporary identifier (C-RNTI), which becomes the (non-temporary) C-RNTIif contention resolution is successful. The contention resolutioninformation is contained within a UE Contention Resolution Identitycontrol element. If the UE Contention Resolution Identity received atthe terminal device from the base station matches a CCCH (Common ControlChannel) SDU (Service Data Unit) transmitted in the Message 3, then theterminal device considers that contention resolution has been successfuland that the random access procedure has been successfully completed.The reason for providing contention resolution is that more than oneterminal device may attempt to access the network using the same randomaccess preamble on the same time and frequency resource. The CCCH SDUstransmitted by the contending terminal devices can be expected to bedifferent and therefore UEs can determined if their random accessresponse is successful by comparing their transmitted CCH SDU with theone sent back to them by the base station in the contention resolutionidentity MAC control element.

Referring back to stage A, a series of random access preambles may betransmitted by the terminal device at progressively higher power levels(power ramping) if a random access response is not received. It shouldbe noted however that each successive random access preamble will bedifferent.

In order to utilise the random access procedure to identify whether aterminal device attempting to connect to the base station needs to beserved by a relay device, the relay device could either:

-   -   Report the RA-RNTIs (Random Access Radio Network Temporary        Identifier) of all random access attempts it sees to the base        station (DeNB) in a new MAC message. The base station would then        determine if there were any random access procedures that the        MTC-RN observed but it did not receive; or    -   Receive all PRACH (Physical Random Access Channel) preamble        attempts and then wait to receive a matching random access        response. Only those attempts for which it fails to get a        response are reported to the base station.

However there are significant problems with both of these mechanismsassociated with the fact that there is no UE context contained in thePRACH preamble and with the power ramping that occurs on the PRACHpreamble. Problems include:

-   -   Due to the power ramping mechanism a PRACH preamble with no        response does not necessarily mean that the terminal device        requires the relay device to access the network. The terminal        device should be allowed to continue up to its maximum number of        RACH preamble attempts (or when maximum power on preamble is        reached) and only then should it be considered that the terminal        device should be served by the relay device;    -   Because subsequent preamble retransmissions use a completely new        randomly drawn RACH preamble, it is difficult or impossible to        determine whether two random access preambles originate from the        same terminal device; and    -   There is no way of knowing whether the RACH preamble has been        transmitted by a MTC-UE or a non-MTC UE.

FIG. 6 illustrates these problems. In FIG. 6, a terminal devicetransmits a series of random access (RACH) preambles P₁, P₂, P₃, P₄ andP₅. These successive transmissions represent totally new randomlyselected RACH preambles after each failure, with power ramping beingapplied. Each of P₁, P₂, P₃ and P₄ are received by the relay device, butare not received by the base station. The relay device can identifyfailures by noting received RACH preambles without a corresponding RACHresponse using the RA-RNTI, but it cannot know that they are originatingfrom the same terminal device. In the present case P₅ is received by thebase station, meaning that the relay device is not required by theterminal device in communicating with the base station. A RACH responseis transmitted by the base station, and could be detected by the relaydevice, informing the relay device that it is not required in relationto the terminal device originating P₅. However, the relay device isunable to correlate the successful RACH preamble P₅ with the failedpreambles P₁ to P₄ and the originating terminal device to identify thatit is not required in relation to those failed preambles.

In order to alleviate these problems, when the terminal device reachesits maximum power in the power ramping of random access preambles, aspecial group of RACH preambles are used instead of the conventionalgroup. The RACH preamble space is partitioned, and currently groups Aand B are used as defined in section 5.1.2 of 3GPP TS36.321. To supportthe present technique, an additional new group, a Maximum Power Group,may be specified. When the terminal device reaches maximum power in thetransmission of random access preambles, for example when a PRACHpreamble retransmission is transmitted at the same power as a previousPRACH preamble transmission, then the terminal device selects a preamblefrom the new Maximum Power Group. The transmission of a preamble takenfrom the Maximum power group effectively notifies the relay device thatthe originating terminal device has failed to connect directly to thebase station. This is shown in FIG. 7.

As with FIG. 6, in FIG. 7 the terminal device chooses RACH preambles R₁,R₂ and R₃ as defined conventionally. As these RACH preambles are usedand fail the terminal device ramps up the transmission power. In eachcase a new randomly chosen RACH preamble is used from the conventionalset of preambles. These RACH preambles are ignored by the relay device.

The terminal device reaches the maximum RACH preamble power withpreamble R₃. Note that (as will be discussed below) this maximum may bemodified from the current specification which allows the terminal deviceto use all available power for preamble transmission. System informationwill define a new preamble group to be used when the new maximum RACHpreamble power is reached. The terminal device selects a preamble R₄from the new Maximum PRACH Power Group and transmits the RACH preambleR₄. Preferably the preamble selected from the Maximum power group istransmitted at full power (either the same power as the final preamblefrom the conventional preamble group, or the maximum transmission powerof the terminal device). This enables the connection to be made morequickly, in view of the fact that there has already been a delay due tothe power ramping with the conventional preambles, and in view of thefact that use of the relay device rather than direct transmission willitself give rise to a further delay. In the alternative, power rampingcould also be used with the relay specific preambles (either with thesame profile as the conventional preambles or with a modified powerramp). In this case the “Maximum Power Group” would instead be a“Relayed Preamble Group”.

Previously, the power ramping of conventional preambles continued up tothe maximum transmission power of the terminal device. However, it ispossible that this might result in a scenario in which the preamble issuccessfully received at the base station but messages on UL-SCH (UplinkShared CHannel) might fail due to a possible sensitivity differencebetween PRACH (used to transmit the random access preambles) and PUSCH(Physical Uplink Shared CHannel) transmission.

In order to alleviate this problem, the PRACH preamble power controlalgorithm can be modified to limit the power of the PRACH preamble forthe terminal device to be capped a number of decibels below the maximumtransmission power of the terminal device.

The relay device detects the use of a RACH preamble R₄ in the MaximumPRACH Power Group, and determines the following:

-   -   The RA-RNTI, based on the first subframe of the PRACH index        within the subframe (in other words the time/frequency resources        used to transmit the RACH preamble);    -   The RAPID, i.e. the RACH preamble ID;    -   The required timing advance for the terminal device so that it        can correctly align transmission of the subsequent Message 3        which will be sent on UL-SCH (this can be determined at the        relay device from the propagation time delay of the RACH        preamble transmission from the terminal device to the relay        device); and optionally    -   The power offset needed by the terminal device for Message 3 to        be received at the relay device at an appropriate reception        power level (this is optional, and may not be required for        example if power ramping is applied to the relay preambles). The        power offset can be determined at the relay device from the        received signal power of the RACH preamble transmission.

The relay device requests time and frequency resources from the basestation in the same way as a conventional terminal device (note therelay device has a different RNTI which is separate from that of theterminal device it is relaying). Subsequently the relay device willreceive an uplink grant via a PDCCH (Physical Downlink Control Channel)message.

A new MAC control element (‘Relayed preamble’ control element) is usedto transfer the RA-RNTI, the RAPID, the timing advance and the poweroffset to the base station via the resources allocated to the relaydevice by the uplink grant.

The base station responds on the PDCCH using the RA-RNTI contained inthe relayed preamble message which allocates PDSCH resources used totransport a (potentially modified) MAC RAR (Random Access Response)message. Note that as introduced above it would be beneficial for theRAR message to be modified to include a power control element whichprovides an offset from the power that the terminal device used for thelast transmitted RACH preamble transmission to be used for thetransmission of message 3. The reason for this is that the currentmechanism whereby the terminal device transmission power for message 3is based on a fixed offset (transmitted in system information) from thepower used for successful preamble power is not applicable because thetransmission power of the random access preamble of the maximum powergroup may be much higher than that required to reach the relay device,or alternatively may be merely adequate. A fixed offset worksconventionally because the power ramping results in a successfulpreamble being transmitted at a power level close to that required. Inthe present case the offset is based on the power offset specified inthe new MAC message. In other ways the random access response is thesame as that currently defined, however it is populated from thecontents of the relayed preamble message rather than based on the directreception of the RACH preamble.

The terminal device receives the RAR and acts in the same way ascurrently specified (with the exception of the power control informationif it is included in the message). Thus it sends a Message 3 response inthe allocated uplink resources. Meanwhile the relay device may alsoreceive the PDCCH component of the RACH response and will subsequentlysetup to receive the (potentially modified) MAC RAR contained in theallocated PDSCH resources in order to obtain the uplink grantinformation and so setup its receivers so that it can receive thesubsequent Message 3. Note that it could be regarded as undesirable forthe relay device to have to monitor messages ultimately not intended forit. In this case a separate new form of uplink grant could be signalledto the relay device. This new form of uplink grant would not actuallyallocate uplink resources but would indicate to the relay device that aparticular terminal device which is to be relayed by the relay devicehas been allocated uplink resources that the relay device needs toreceive. An example of such a mechanism is described further below.

The relay device receives the Message 3 from the terminal device (thisshould be correctly timing advanced and, optionally, correctly powercontrolled). The Message 3 will not be received by the DeNB, which isout of transmission range.

The MTC relay device encapsulates the Message 3 received from theterminal device within a ‘relayed UL message’ in MAC. The header of thismessage will contain a TPC (Transmit Power Control) and TA (timingadvance) indication. The relay device requests uplink resources from thebase station in order to send the encapsulated message.

The relayed UL message is received by the base station which extractsthe encapsulated Message 3 and goes on to complete contention resolutionas described previously.

For requesting resources for the terminal device from the base stationbased on the random access preamble, a new MAC control element, theRelayed preamble message, is provided, with a suggested format beingshown in FIG. 8. As can be seen from FIG. 8, the relayed preamblemessage comprises a reserved part (R), a timing advance part (TAcommand), a RACH preamble ID part (RAPID), a power offset (Power controlinfo) a Random Access Temporary Identifier (RA-RNTI) and optionalpadding (PAD). These elements have all been discussed above, with theexception of the padding which is used to pad the message to a requiredsize.

A modified RAR message is schematically illustrated in FIG. 9. Inparticular, the modified RAR message comprises a reserved part (R), atiming advance part (TA command), an allocation of uplink time andfrequency resources (UL grant), the further temporary identifier(C-RNTI), a power offset (Power control info) and optional padding(PAD). This is the same format as previously used for RAR, but with apower control info field added.

In addition, there may need to be a change to the RRC IE‘RACH-ConfigCommon’ to include a new group C set of preambles. Note thatthe signalling currently defined is such that this change would bebackwardly compatible to Release 8 LTE.

Relay Arbitration

There may be multiple relay devices which receive the random accesspreamble in the Maximum PRACH Power Group transmitted by the terminaldevice. Accordingly, the base station may receive multiple relayedpreamble MAC messages for the same terminal device from different relaydevices. To address this, a mechanism is required to determine which ofthe relay devices is to become the serving relay device for the terminaldevice.

FIG. 10 schematically illustrates this. In FIG. 10, a first relay devicetransmits a relayed preamble MAC message to the base station. A secondrelay device also transmits a relayed preamble MAC message to the basestation. The base station determines which relay device to use to servethe terminal device, and transmits a serving relay indication MACmessage indicating that the recipient relay device is to be the servingrelay device for the indicated terminal device. In one example, the basestation examines power control information in the relayed preamble MACmessage and determines which relay received the preamble transmissionfrom the terminal device most strongly. The relay receiving the preambletransmission most strongly is then selected to be the serving relaydevice for that terminal device. It will be appreciated that otherfactors could also be taken into consideration, such as relative trafficloading between the relay devices.

A schematic flow diagram is shown in FIG. 11 which demonstrates anexample method of arbitrating between multiple relay devices. Firstly,at a step A1 the base station (DeNB) starts collecting relayed preambleMAC messages from the relay devices (MTC-RNs). A number of MTC-RNs seethe same PRACH preamble transmitted in the Maximum PRACH Power Group andthus multiple relayed preamble MAC messages are received by the DeNB.These messages will indicate the same RA-RNTI and RAPID but are likelyto indicate different power control information fields. At a step A2,after a defined time interval (T_(collect)) the DeNB stops collectingrelayed preamble MAC messages from the MTC-RNs. A typical defined timeinterval is expected to be approximately 1 second. Once this has expireda decision can be taken regarding which MTC-RN is to be the serving nodebased on the power control information in the received relayed preamblemessages. Specifically, at a step A3, the DeNB compares all receivedpower control information fields for the relayed preamble MAC messages,and at a step A4 selects the MTC-RN with the best signal level to be theserver for the MTC-UE in question.

If multiple power control information fields from different MTC-RNsindicate the best signal level (within a predetermined decibel range),then the base station selects between these at a step A5 by taking intoaccount the number of associated terminal devices for each relay device.In particular, the base station then preferentially selects the relaydevice (of those having the best signal level) having the lower numberof associated MTC-UEs (load balancing). Finally, at a step A6 the DeNBsends a serving relay indication with payload ACK to the selectedMTC-RN, and payload NACK to all other MTC-RNs. This may be achieved byway of a new message ‘Serving relay indication’ MAC control element,which will indicate that the UE corresponding to the current RA-RNTI(and RAPID) will be controlled by the recipient MTC-RN of that message.A NACK indication will be sent to all other MTC-RNs. A possiblerealization for sending the NACK indication is via a group RN-RNTImechanism. This means that the NACK indication would be targeted in someway to all MTC-RNs which originated a relayed preamble MAC messagerelating including the same RA-RNTI. A possible realisation forindicating a NACK to all associated MTC-RNs is therefore the use of aPDSCH allocation scrambled with that RA-RNTI. It is recognized that theselected MTC-RN would receive both an ACK and a NACK, so logically inthis case the MTC-RN would interpret this to mean ACK. It should also beappreciated that the use of a group RNTI mechanism to address multipleMTC-RNs could be of more general applicability, and is not simplylimited to the transmission of NACK messages.

A possible format for the ‘Serving relay indication’ MAC control elementis shown in FIG. 12. This MAC control element comprises a Random AccessTemporary Identifier (RA-RNTI) as contained in the relayed preamble MACcontrol element, a relay device indicating field (S) which is a 1 bitfield with ‘1’ indicating that the relay device is the serving node and‘0’ indicating it is not the serving node, a RACH preamble ID part(RAPID) as contained in the relayed preamble MAC control element, and areserved part (R).

Once an MTC-RN has been selected as a serving MTC-RN for a given MTC-UE(either because it was the only MTC-RN to relay the random accesspreamble or because DeNB arbitration selected it to be the serving relaynode out of two or more candidates) then an entry for that MTC-UE willbe made in a list of served MTC-UEs stored at the MTC-RN. The list wouldinclude the C-RNTI of the MTC-UE (used to identify and accesscommunications between the DeNB and the MTC-UE), as well as some form ofpower control context information. This power control contextinformation could include a target channel interference between theMTC-RN and the MTC-UE so that TPC (Transmit Power Control) commands canbe generated.

For the MTC case it is reasonable to assume that UEs will have a lowmobility, and so once the UE has connected via the MTC-RN it will stayassociated with the same MTC-RN. For the mobility case a UE (e.g.non-MTC) can be added to the list when the DeNB informs the MTC-RN thatit has been selected. The UE can be removed from the list when the UEdisconnects the RRC connection.

MAC Relaying

As part of uplink-only relaying operation the relay device receives MAC(Media Access Control) Protocol Data Units (PDUs) from the terminaldevice and adds a new header termed ‘MAC relay header’ to the MAC PDUs.

Schematic illustrations of downlink and uplink end-to-end protocolstacks for uplink-only relaying are provided in FIGS. 13 and 14respectively.

FIG. 13 illustrates the downlink (base station to terminal device)end-to-end protocol stack. Protocol stacks are shown at the terminaldevice (MTC-UE), the relay device (MTC-RN) and the base station (DeNB).The MTC-RN is shown with two protocol stacks, as both transmit andreceive functions for the MTC-RN are shown. The MTC-RN is likely tocommunicate with only one DeNB, but may communicate with multipleMTC-UEs. In order to reflect this, the DeNB protocol stack and the righthand protocol stack in the MTC-RN (that is, the protocol stack used tocommunicate with the DeNB) are shown as single stacks (single instance),whereas a plurality of parallel protocol stacks are shown for the MTC-UEand the left hand protocol stack of the MTC-RN (that is, the protocolstack used to communicate with the MTC-UEs). The protocol stacks for theMTC-UE, the DeNB and the right hand stack of the MTC-RN have thefollowing layers (from the highest layer to the lowest layer):

-   -   RRC (radio resource control);    -   PDCP (packet data convergence protocol);    -   RLC (radio link control);    -   MAC (medium access control); and    -   L1 (physical layer).

Control and data signalling for transmission from a transmitting deviceto a receiving device starts at a high layer (for example the RRClayer), is repeatedly transformed or processed through the various lowerlayers to the physical layer, and is then transmitted to the receivingdevice at the physical layer. At the receiving device the controlsignalling is received at the physical layer and then propagates upthrough the protocol stack typically to the layer at which it originatedin the transmitting device, for example the RRC layer.

Two signal flows are shown in FIG. 13. The first of these is an MTC-UEdata and control signal flow from the DeNB to the MTC-UE. Thispropagates down through the protocol stack at the DeNB and istransmitted directly to the MTC-UE at the physical layer (L1). As willbe understood, downlink transmission from the DeNB to the MTC-UE iscarried out directly, and not via the MTC-RN. The MTC-UE data andcontrol signal flow, when received at the MTC-UE, propagates back upthrough the protocol stack at the MTC-UE. The second signal flow is anMTC-RN control signal flow from the DeNB to the MTC-RN. This propagatesdown through the protocol stack at the DeNB and is transmitted directlyto the MTC-RN at the physical layer (L1). The MTC-RN control signalflow, when received at the MTC-RN, propagates back up through theprotocol stack at the MTC-RN. For the downlink case, the left handprotocol stack at the MTC-RN (used for communicating with the MTC-UE) isnot utilised, because the MTC-RN is not used to communicate with MTC-UEson the downlink.

FIG. 14 illustrates the uplink (terminal device to base station)end-to-end protocol stack. Protocol stacks are shown at the MTC-UE, theMTC-RN and the DeNB in the same configuration as FIG. 13. Explanation ofthe layer structure will therefore not be repeated here. As with FIG.13, multiple MAC instances are provided between the MTC-RN and apotential plurality of MTC-UEs.

Two signal flows are shown in FIG. 14. The first of these is an MTC-UEdata and control signal flow from the MTC-UE to the DeNB. Thispropagates down through the protocol stack at the MTC-UE and istransmitted to MTC-RN at the physical layer. The control signal flowthen propagates up to the MAC layer in the MTC-RN, where it is processedat the MTC-RN (in a manner to be described below) and then propagatedback down to the physical layer and retransmitted to the DeNB at thephysical layer. At the DeNB the MTC-UE data and control signallingpropagates back up through the protocol stack. It will be appreciatedfrom this that the MTC-UE data and control signalling is processed atthe MTC-RN at the MAC layer. For this reason, the upper portion of theprotocol stack for receiving data from the MTC-UE can be omitted,permitting simplification of the MTC-RN. The second signal flow is anMTC-RN control signal flow from the MTC-RN to the DeNB. This propagatesdown through the protocol stack at the MTC-RN and is transmitteddirectly to the DeNB at the physical layer (L1). The MTC-RN controlsignal flow, when received at the DeNB, propagates back up through theprotocol stack at the DeNB. It will therefore be appreciated that a fullprotocol stack is required at the MTC-RN for controlling thecommunications link between the MTC-RN and the DeNB.

From FIGS. 13 and 14 it will be appreciated that processing is conductedat the MAC layer. In the MAC layer, data is communicated in MAC ProtocolData Units (PDUs) on the DL-SCH or the UL-SCH. Each MAC PDU comprises aMAC header, zero or more MAC Service Data Units (SDUs), zero or more MACcontrol elements, and optionally padding. The MAC header may consist ofmultiple sub-headers, each corresponding to one of a MAC controlelement, a MAC SDU, or padding. The sub-headers contain certaininformation describing the contents and size of the respective MACcontrol element, MAC SDU, or padding. The header and sub-headers maycomprise an LCID (Logical Channel Identifier) field which indicateseither a logical channel for a corresponding MAC SDU, or a type of thecorresponding MAC control element. In FIGS. 13 and 14, MAC PDUs (ofvarious different types) are transmitted from the MTC-UE and the MTC-RNon the uplink, between the MTC-RN to the DeNB on the uplink anddownlink, and between the DeNB and the MTC-UE on the downlink.

When a MAC PDU is transmitted from the MTC-UE to the MTC-RN, the MTC-RNadds the new ‘MAC relay header’ to the MAC PDUs (which could at thisstage be considered to be SDUs because they serve as an input into MAClayer processing) and transmits the resulting MAC relayed PDU to theDeNB. This is schematically illustrated in FIG. 15. In FIG. 15, anMTC-UE 310 transmits a MAC PDU 315 to an MTC-RN 320. At the MTC-RN 320,the received unsegmented MAC PDU 315 is provided with a relay header 323which identifies the MTC UE 310 (UE identifier—for example the C-RNTI)and provides timing advance (TA) and power control information(typically a TPC decision). The timing advance information is derived bythe MTC-RN based on a propagation time delay of the transmission fromthe MTC-UE to the MTC-RN. Similarly, the power control information isderived by the MTC-RN based on the reception power level of thetransmission from the MTC-UE to the MTC-RN. The timing advance and powercontrol information are required by the MTC-UE in setting a transmissiontime and transmission power of a subsequent transmission to the MTC-RN.This information cannot be provided directly from the MTC-RN to theMTC-UE because the MTC-RN does not communicate with the MTC-UE on thedownlink (uplink-only relay). In order that the MTC-UE can ultimately beprovided with this information, it is specified in the relay header 323.Conventional R/R/E/LCID/F/L and R/R/E/LCID elements (see section 6.1.2of TS 36.321) may be used to indicate the existence of the MAC relayheader control element and the size of the associated encapsulated MACPDU from the MTC-UE. In addition, conventional MAC control elements 322such as BSR (buffer status report) can be added, as defined in TS36.321. It should be noted that a new LCID value for the MAC relayheader may also be required. It should also be noted that the receivedMAC PDU from the MTC-UE becomes a MAC SDU in the MTC-RN (encapsulation),and that multiple MAC PDUs from a plurality of MTC-UEs can betransmitted in a single MAC PDU from the MTC-RN. Thus concatenation ispossible; however segmentation of MAC PDUs from MTC UEs may not be.

To explain the difficulty in providing segmentation at the MTC-RN, itshould be understood that, at an MTC-UE, physical resources areallocated for transmitting uplink data, and these physical resourcesrepresent a certain number of data bits that can be transported.

At the MTC-UE, the MAC layer determines, once headers have been takeninto account, the amount of data that can be placed in the allocatedresources and it asks the upper layer (which is the RLC) for a packet ofthis size (this would be referred to as a MAC SDU or RLC PDU, an SDUbeing the input to a layer while the PDU is the output of a layer). TheRLC is responsible for splitting buffered packets into usually smaller(but not necessarily so) packets that are to be transmitted over the airinterface. If big packets are being split into smaller packets then thisis referred to as segmentation. As will be appreciated from the above,this is conventionally carried out by the RLC. Once this data hasarrived at the MAC layer it will match (in size) the allocation and canthen be transmitted on the uplink.

The problem for the MTC-RN at MAC level is that there is no means ofperforming segmentation at the MAC layer (and we have no RLC layer to dothis job for us), and so what could happen is illustrated in thefollowing simple example:

The MTC-RN receives 2 MAC PDUs from UEs that are to be relayed on to theDeNB. Each of them is 100 bits in size and they are from separate UEs.The MTC-RN informs the DeNB of it's buffer occupancy—200 bits. There isno obligation for the DeNB to grant an allocation which will transferall of this data so an allocation of 190 bits may for example be made.We have no means of segmentation at the MAC layer, so the only optionthe MTC-RN has is to send one of the MAC PDUs that need to relayed andwaste the rest of the allocation. The situation could be even worse ifthe DeNB grants an allocation of less than 100 bits, because the MTC-RNis then unable to transmit anything.

If this were to be a major problem then it would be possible to providenew functionality in MAC specifically for relay nodes that providessegmentation functions. However, this would result in an undesirableincrease in complexity, and would have little advantage over relayinginstead at the RLC level.

However, the lack of segmentation functionality at the MTC-RN may not beunduly problematic because:

-   (1) The radio link between the MTC-RN and the DeNB is likely to be    much better than that between the MTC-RN and the MTC-UE. So it's    relatively unlikely that a scenario where the MTC-RN can send less    data than the MTC-UE has sent to the relay DeNB would arise; and-   (2) The scheduler in the DeNB is aware that the MTC-RN is a relay    node and it can have a reasonable idea of the size of the PDUs that    are to be relayed by the MTC-RN (because it has previously allocated    resources for these PDUs to MTC-UEs being served by the MTC-RN).    Accordingly, the scheduler can allocate the MTC-RN a high priority    and allocate uplink resources to the MTC-RN such that the    inefficiency discussed above does not occur, or is at least    mitigated.

The MTC-RN will always send a MAC relay header control element even ifit fails to receive the MAC PDU from the MTC-UE. This mechanism providesan implied ACK/NACK to the DeNB. It will be appreciated that the MTC-RNwill be aware that a MAC PDU from an MTC-UE has failed to arrive becauseit is aware that uplink resources have been allocated to the MTC-UE, andso can expect an uplink transmission from the MTC-UE at a particulartime and frequency resource.

The MAC PDU 315 from the UE, the relay header 323 and the conventionalMAC control elements 322 together make up a MAC relayed PDU 325 which istransmitted from the MTC-RN 320 to the DeNB 330.

At the DeNB 330 the following processing occurs:

The DeNB 330 receives the relayed MAC PDU 325 and acts as if this wasnormally received from a conventional UE. In particular, at a MACprocessor 332, ACK/NACK indications are obtained in relation to theMTC-RN 320, TPC commands are generated in relation to the MTC-RN 320,and in the MAC layer the MAC control elements 322 are acted on. It willbe appreciated that these parameters and actions are directed at theMTC-RN 320.

At the DeNB 330, a relay demultiplexer 334 then uses the contents of theMAC relay header 323 to control timing advance control to the MTC UE310. Also power control TPC commands are generated based on the powerinformation contained in the header 323. ACK/NACK indications are basedon the presence or absence of a MAC PDU 315 from the MTC UE 310 withinthe MAC relayed PDU 325. This could be determined directly, or from thecontents of a P field in the MAC relay header, as explained below.

The relay demultiplexer 334 also extracts the MAC PDU 315 originallytransmitted by the MTC-UE 310 from the MAC relayed PDU 325 and returnsthis to the MAC processor 332 for processing. The C-RNTI contained inthe MAC relay header control element is used to identify the UE contextin the MAC for processing the MAC PDU 315. Regular processing of the MACPDU 315 then occurs and ultimately a MAC SDU 338 corresponding to thedata originated from the MTC-UE 310 is sent to higher layers of theprotocol stack.

A suggested format for the MAC relay header control element is shown inFIG. 16. In FIG. 16, C-RNTI is the relayed UE's C-RNTI, TPC is the TPCdecision made at the relay device for the MTC UE, TA info is the timingadvance info obtained at the relay device for the MTC UE, and Pindicates the presence or absence of an associated MAC PDU. If theassociated MAC PDU was received correctly then this bit will be set to‘1’ and the MAC PDU will be present in the payload of the MAC relayedPDU (i.e. there was a ACK). If set to ‘0’ then no MAC PDU is present(i.e. there was a NACK).

An example of the new MAC relayed PDU structure is shown schematicallyin FIG. 17 for a case where all of the scheduled transmissions from theMTC-UEs are correctly received by the MTC-RN.

A MAC header 410 of the PDU comprises a number of sub-headers 412, 414and 416. The sub headers 412 correspond to respective control elements420, 430 in the MAC payload. The sub headers 414 correspond to SDUs 440in the MAC payload, and the sub head 416 corresponds to optional padding450 within the MAC payload. Generally, the MAC header 410 is ascurrently defined in the LTE specification except that a new LCID valueis available for use in R/R/E/LCID subheaders 412 so that associated MACrelay header control elements 430 can be signalled. The MAC controlelements should preferably be ordered as follows:

Conventional MAC control elements 420 should be specified first. Notethat in FIG. 17 m conventional control elements 420 are provided.

MAC relay header control elements 430 should be placed after theconventional MAC control elements 420. There will be a MAC relay controlelement 430 for each MAC PDU to be relayed and the order of the MACrelay header control elements 430 will be the same as the order of theMAC PDUs to be relayed (for this case where all P fields are set to‘1’). Note that in FIG. 17 there are n MAC relay header control elements430 and n MAC PDUs to be relayed.

If a single MAC relayed PDU contains multiple received MAC PDUs from thesame MTC-UE then the first received should be the first in the overallconstructed MAC relayed PDU, the second the second and so on.

The MAC SDUs 440 in FIG. 17 are actually MAC PDUs that have beenreceived by the MTC-RN 320 and are to be relayed (they contain a MACheader, MAC control elements and a MAC SDU which has been transmittedfrom the MTC-UE 310). Note that in FIG. 17 there are n MAC SDUs 440.

As can been in FIG. 17 there are multiple MAC SDUs 440 in theconstructed MAC relayed PDU. As discussed above each one of the theseMAC SDUs 440 is in fact a MAC PDU transmitted from a MTC-UE and receivedby the MTC-RN. These can come from either a single UE (where thereceived MAC PDUs have been stored over multiple transmission timeintervals (TTI)) or from multiple different UEs. Thus a form ofconcatenation of multiple MAC PDUs from MTC-UEs to form a single MACrelayed PDU is possible. However it should be noted segmentation of theMAC PDUs received from MTC-UEs may not be possible.

An example of the new MAC relayed PDU structure is shown schematicallyin FIG. 18 for a case where some of the scheduled transmissions from theMTC-UEs are not correctly received by the MTC-RN. A scheduledtransmission may be considered as not correctly received if either it isnot received at all, or if it was incorrectly received (corrupted orincomplete).

In FIG. 18, there are still n MAC relay headers in a MAC header 510,because the MTC-RN is reporting on the same number of scheduled MTC-UEtransmissions. As with FIG. 17, a set of conventional MAC controlelements 520 are present, as are a set of MAC relay headers 530, MACSDUs 540 and optional padding 550. In FIG. 18, some of the MAC relayheader control elements 530 have their P field set to ‘0’ to indicatethat a MAC-PDU was not correctly received for an associated UL grant. Itis assumed that there are p of these cases. For a MAC relay headercontrol element 530 with P field set to ‘0’ there is no associated MACSDU. Thus the total number of MAC SDUs 540 (MAC PDUs actually receivedfrom MTC-UEs) is n-p. The order of the MAC relay headers 530 which haveP set to ‘1’ should correspond correctly with the MAC SDUs 540 containedin the MAC relayed PDU. Again, if a single MAC relayed PDU reports onmultiple MAC PDU from a single MTC-UE, the order of the MAC relaycontrol elements and the associated MAC SDUs (although these might notall be present) should be in order that they are scheduled to bereceived.

A suggested new LCID for the MAC relay header control element is shownhighlighted in table 1 below (the non-highlighted values are the valuescurrently defined in the LTE specification).

TABLE 1 A suggested new LCID for the MAC relay header control element.Index LCID values 00000 CCCH 00001-01010 Identity of the logical channel01011-11000 Reserved 11001 MAC relay header 11010 Power Headroom Report11011 C-RNTI 11100 Truncated BSR 11101 Short BSR 11110 Long BSR 11111PaddingRelay Control

A procedure for handling the relaying of UE data by the relay followingthe random access procedure is now described with reference to FIG. 19.When an MTC-UE wishes to send data to a DeNB it transmits a SchedulingRequest (SR) on the PUCCH on a previously allocated time/frequencyresource. This is typically a single bit (transmitted or nottransmitted) within the allocated time/frequency resource. The DeNB isresponsive to receiving the SR to allocate and signal an uplink grant tothe MTC-UE.

Request for Resource from the MTC-UE

If no PUCCH (Physical Uplink Control Channel) resources have beenassigned to the UE then it will need to conduct a random accessprocedure, this is described above in detail and indicated in FIG. 19.

Once the random access procedure is complete the MTC-UE sends a SR(Scheduling Request) on the allocated PUCCH resource. The MTC-RN willhave been informed of the PUCCH resources that the MTC-UE has beenallocated and attempts to receive PUCCH transmissions at theseinstances. Two mechanisms by which the MTC-RN could be made aware of thePUCCH resources allocated to the MTC-UE are as follows:

-   (1) Directly after the random access procedure, the PUCCH resources    will have been assigned to the MTC-UE in an RRCConnectionSetup RRC    message which is sent unencrypted by the DeNB. It is therefore    possible for the MTC-RN to monitor the PDCCH and the PDSCH for this    RRC message and use the configuration information provided by this    message to set up to receive a PUCCH transmission from the MTC-UE;    or-   (2) A new RRC message could be used to inform the relay node of the    PUCCH resource assigned to the MTC-UE it is serving. The new RRC    message could be transmitted directly to the MTC-RN by the DeNB, for    example on the PDCCH and/or the PDSCH. The advantage of this option    over the first option is that with the first option if the PUCCH is    reconfigured then the MTC-RN may not be able to determine this    because subsequent RRC messages are encrypted.

When a PUCCH transmission is received by the MTC-RN in relation to anMTC-UE for which it is responsible, it generates a new MAC controlelement ‘Relayed SR message’ which contains a C-RNTI of the MTC-UE. Notethat the MTC-RN may request and be granted uplink resources in order tosend the relayed SR message to the base station in the same way as aconventional UE. It will be appreciated that there are other options forrelaying the SR to the base station, for example using physical layer(L1) signalling, noting that the SR is generally a single 1 bit flag.The DeNB is responsive to the SR to allocate uplink resources to theMTC-UE. This uplink grant is communicated directly to the MTC-UE on thePDCCH. As will be explained further below, there may also be an optionaladditional separate message indicating (1) the MTC-UE uplink allocationto the relay device and (2) a subsequent uplink allocation for theMTC-RN to use in sending the encapsulated MTC-UE message to the DeNB.

General Mechanism for Relaying Uplink MAC PDUs from the MTC-UE

The general mechanism for relaying uplink MAC PDUs from the MTC-UE isnow described. In FIG. 19 the BSR (Buffer Status Report) is transferred(this is the standard case for a first transmission after the SR inorder to advise the DeNB how much data there is to send on the uplinkand thus how much future uplink resource is to be required) butembodiments of the present invention apply similarly to user plane data.

There are a number of possible options described below; each depends onthe level of inference (and the amount of signalling) that is made atthe MTC-RN.

Option 1: Separate PDCCH Grant to MTC-UE and Indication of this Grant tothe MTC-RN with No Implied Grant to MTC-RN.

In this case the following procedure occurs, as shown in FIG. 20:

At a step A1, a grant of UL resources is made to the MTC-UE andcommunicated on the PDCCH. At around the same time, at a step A2, aseparate grant is made to the MTC-RN which is different from aconventional grant because it does not actually allocate PUSCH resourcesbut informs the MTC-RN that the MTC-UE has been allocated PUSCHresources. This allows the MTC-RN to configure its receiverappropriately. Note that this grant can take one of 2 forms:

-   -   1. A new PDCCH format which contains a modified uplink grant        (certain parameters such as new data indicator are not needed)        and a mechanism to indicate the UE that the allocation is being        made to; or    -   2. A conventional PDCCH grant of downlink resources to the        MTC-RN is made and a new MAC control element is sent to the        MTC-RN on the subsequent PDSCH transmission which indicates the        uplink resources which have been sent to the UE in question        (identified by its C-RNTI). This mechanism is similar to the MAC        random access response procedure, although there is no actual        uplink grant to the MTC-UE, just an indication that a grant has        been made to a particular UE.

At a step A3, the MTC-UE transmits on the PUSCH in the granted resourcesand this is received by the MTC-RN (the DeNB does not receive this PUSCHtransmission). At a step A4, the MTC-RN acts in the same way as a UEwith data newly arrived in its buffers and requests from the basestation, and is subsequently granted (at a step A5), UL resources. At astep A6, the MTC-RN adds an uplink relay MAC header (see above), whichcontains power control and timing advance information, to the receivedMAC PDU from the MTC-UE and transmits this to the base station in theallocated resources. Note also that if the MTC-RN did not receive theMAC PDU from the MTC-UE the MTC-RN still sends an uplink relay MACheader but with no encapsulated MAC message from the MTC-UE. Followingthe step A6, the DeNB receives the data and a new MAC sub-layer is usedto remove the header and determine the MTC-UE from which it ultimatelycame. Note that there is also timing advance and power controlinformation in the MAC header. ACK/NACK is implied from the presence orabsence of the encapsulated MAC message from the MTC-UE (possibly basedon a “presence” bit in header information of the MAC message).

FIG. 21 repeats the process of FIG. 20, but schematically illustrateshow the process is modified when the PUSCH transmission from the MTC-UEis not received at the MTC-RN. In particular, the steps A1, A2, A4 andA5 are identical to the identically numbered steps in FIG. 20, and willtherefore not be described again. A step A3′ differs from the step A3 inFIG. 20 because the transmission on PUSCH never reaches the MTC-RN (oralternatively arrives incomplete or corrupted). In this case, the MTC-RNstill requests resources and sends a PUSCH transmission to the basestation, but in this case there is no MTC-UE originating contentincluded in the PUSCH transmission. This procedure effectively informsthe base station that the scheduled transmission from the MTC-UE hasfailed.

Option 2: Single PDCCH Grant to MTC-UE with No Implied Grant to MTC-RN

In this case the following procedure occurs, as shown in FIG. 22:

At a step B1, a grant of UL resources is made to the MTC-UE andcommunicated to the MTC-UE on the PDCCH. No separate indication of thesegranted resources is sent to the MTC-RN. The MTC-RN is required tomonitor the PDCCH to detect messages associated with this MTC-UE (at astep B2). The MTC-RN is able to achieve this by performing a CRC (CyclicRedundancy Check) mask on the PDCCH not only for its own C-RNTI(standard technique for a device to obtain PDCCH data intended for it),but also for the C-RNTI of the MTC-UE, which the MTC-RN is aware of (forexample via the Random Access Response described above). In this way,the MCT-RN is able to identify that a grant has been provided to theMTC-UE and configure its receivers appropriately. The rest of theprocedure is as defined in Option 1, with steps B3, B4, B5 and B6corresponding to the steps A3, A4, A5 and A6 of FIGS. 22 and 23, thedescription of these therefore not being described here.

Option 3: Separate PDCCH Grant to MTC-UE and Indication of this Grant tothe MTC-RN with Implied Grant to MTC-RN

In this case the following procedure occurs, as shown in FIG. 23.

As in Option 1 a grant of UL resources is made to the MTC-UE andtransmitted on the PDCCH at a step C1. As in option 1 a separate grantis made to the MTC-RN at a step C2, which is different from aconventional grant because it does not actually allocate PUSCH resourcesbut informs the MTC-RN that the MTC-UE has been allocated PUSCHresources. This allows the MTC-RN to configure its receiverappropriately. As in option 1 the MTC-UE transmits on PUSCH in thegranted resources at a step C4, and this is received by the MTC-RN (theDeNB does not receive this PUSCH transmission). In contrast to Option 1,the reception of the message by the MTC-RN in step C4 implies a UL grantof resource in a later subframe (or also contains an additional explicitindication) to be used by the MTC-RN to send data to the DeNB. As anoption, the base station may (at a step C3), transmit an MTC-RN PUSCHallocation message on the PDCCH to allocate uplink resources to theMTC-RN for relaying the subsequent PUSCH transmission at the step C4.

At a step C5, the MTC-RN adds an uplink relay MAC header (see below) tothe received MAC PDU from the MTC-UE and transmits this in the allocatedresources. Note also that if the MTC-RN did not receive the MAC PDU fromthe MTC UE it still sends a UL relay MAC header but with no encapsulatedMAC message from the MTC-UE. Following the step C5, the DeNB receivesthe data and a new MAC sub-layer is used to remove the header anddetermine the MTC-UE from which it ultimately came. Note that there isalso timing advance and power control information in the MAC header.ACK/NACK is implied from the presence or absence of the encapsulated MACmessage from the MTC-UE.

Option 4: Single PDCCH Grant to MTC-UE with Implied Grant to MTC-RN

In this case the following procedure occurs, as shown in FIG. 24:

As in option 2 a single grant of UL resources is made to the MTC-UE andtransmitted on the PDCCH at a step D1, which the MTC-RN also receives bymonitoring the PDCCH at a step D2. There is an implied allocation in alater subframe based on this the reception of an allocation to a MTC-UEwhich the MTC-RN knows it is responsible for forwarding data to theDeNB. Subsequent steps D3 (which is optional), D4 and D5 correspond tothe steps C3, C4 and C5 in Option 3, the description of these thereforenot being repeated here.

Comparison of Options

The benefits of Option 1 are as follows:

-   -   The use of separate PDCCH messages for informing the MTC-RN of        the uplink grant to the MTC-UE mean that the MTC-RN is not        forced to monitor the PDCCH to see where the MTC-UE UL grants        are made but is informed directly.    -   The MTC-RN requests and is allocated explicit resources to send        data from it to the DeNB. This has the advantage that it is much        easier to operate efficient rate control for relaying        transmissions from the MTC-RN. In particular, the MTC-RN will        request only the uplink resources it needs to relay the messages        actually received from the MTC-UE. Also, requesting resources        from the base station improves the ease of concatenating MAC        PDUs from multiple MTC-UEs into a single transmission.

The disadvantages of Option 1 are as follows:

-   -   A new allocation mechanism would be required, for example        similar to a RAR which indicates the C-RNTI of the MTC-UE and        the uplink grant to inform the MTC-RN of allocations to the        MTC-UE.    -   The MTC-UE has no implied allocation of resources for it to send        data back up to the DeNB, which gives rise to a delay penalty.

The benefits of option 2 are as follows:

-   -   No separate indication is required to be sent to the MTC-RN to        indicate that the MTC-UE has been granted uplink resources. This        is more efficient than the communication of explicit        indications.    -   The MTC-RN requests and is allocated explicit resources to send        data from it to the DeNB. This has the advantage that it is much        easier to operate efficient rate control for relaying        transmissions from the MTC-RN. In particular, the MTC-RN will        request only the uplink resources it needs to relay the messages        actually received from the MTC-UE. Also, requesting resources        from the base station improves the ease of concatenating MAC        PDUs from multiple MTC-UEs into a single transmission.

The disadvantages of Option 2 are as follows:

-   -   The MTC-RN is required to look at PDCCH messages not intended        for it and for it to set up its receivers to receive the        indicated uplink resources.    -   The MTC-UE has no implied allocation of resources for it to send        data back up to the DeNB, resulting in a delay penalty.

The benefits of Option 3 are as follows:

-   -   The use of separate PDCCH messages for informing the MTC-RN of        the uplink grant to the MTC-UE mean that the MTC-RN is not        forced to monitor the PDCCH to see where the MTC-UE UL grants        are made but is informed directly.    -   The MTC-RN implies an uplink grant from the indication that it        is to receive data from the MTC-UE. This is more efficient in        terms of signalling and reduces delay.

The disadvantages of Option 3 are as follows:

-   -   A new allocation mechanism would be required, for example        similar to a RAR which indicates the C-RNTI of the MTC-UE and        the uplink grant to inform the MTC-RN of allocations to the        MTC-UE.    -   It would be difficult to run an efficient rate control function        and if the MTC-RN fails to receive the transmission from the        MTC-UE there will only be the small relay header to send—this        might require far less resources than actually allocated by the        base station.

The benefits of Option 4 are as follows:

-   -   No separate indication is required to be sent to the MTC-RN to        indicate that the MTC-UE has been granted uplink resources. This        is more efficient than the transmission of explicit indications.    -   The MTC-RN implies an uplink grant from the indication that it        is to receive data from the MTC-UE. This is clearly more        efficient in terms of signalling and reduces delay.

The disadvantages of Option 4 are as follows:

-   -   The MTC-RN is required to look at PDCCH messages not intended        for it and for it to set up its receivers to receive the        indicated uplink resources.    -   It would be difficult to run an efficient rate control function        and if the MTC-RN fails to receive the transmission from the        MTC-UE there will only be the small relay header to send—this        might require far less resources than actually allocated by the        base station.

Overall option 2 probably offers the best choice for MTC devices in anLTE network, but as can be appreciated from the above all of the optionshave their relative merits.

The discussion above highlighted the need for a mechanism to relayscheduling requests (SR) received on PUCCH by the MTC-RN to the DeNB.This is achieved by the use of a new MAC control element, the ‘relayedSR’ MAC control element. A proposed structure for this MAC controlelement is shown in FIG. 25. The relayed SR needs only indicate theMTC-UE's C-RNTI (the LCID in the MAC header will tell the DeNB this is arelayed SR and thus a small uplink allocation is needed to send a BSR).

The new PDCCH approach may use a cut down version of DCI format 0 (CQIrequest, TPC command and new data indicator are not required by theMTC-RN which is only performing reception of the allocated UL resources)together with a relayed UE identifier which is an index into a tablewhich contains the actual C-RNTIs of the MTC-UEs that the MTC-RNs istasked with relaying. This table is signaled to the MTC-RN by(preferably) RRC signalling. This will of course mean that RRC messagingis needed to configure the MTC-RN correctly.

An example implementation of a new MAC control element for indicating tothe MTC-RN on which uplink resources to expect a transmission from theMTC-UE is schematically illustrated in FIG. 26 is termed an ‘MTC-UEallocation’ control element. The MTC-UE allocation control element issimilar to the RAR format but needs its own LCID as it is used to informthe MTC-RN it must turn on its receivers to receive the indicated ULallocation from the MTC-UE. The MTC-UE allocation control elementscomprises a C-RNTI field which specifies the C-RNTI of the MTC UE towhich an uplink grant has been made. A field “UL grant” specifies thegrant provided to the MTC-UE which the MTC-RN should setup to receive.The “R” field is Reserved.

Overall the PDCCH approach is preferable because it requires a singlemessage whereas the new MAC control element approach means that twomessages are needed, one of which is sent on the PDSCH which ispotentially subject to a block error rate (BLER) target (because HARQ isoperating on this channel).

In FIG. 27, a schematic diagram of a wireless communications system 1000comprising a base station (DeNB) 1100, a relay device (MTC-RN) 1200 anda terminal device (MTC-UE) 1300 is shown. The DeNB 1100 comprises atransmitter/receiver part 1120 (transmitter and receiver) whichtransmits and receives data over an air interface using an antenna 1125.The DeNB 1100 also comprises a processor 1140 which controls thescheduling of communications over the air interface as well ascoordinating the operation of the DeNB 1100 with the core networkelements shown in FIG. 2 (not shown here). The DeNB 1100 is connected tothe core network via a backhaul link 1160. The DeNB 1100 communicates inboth the downlink and uplink direction with the MTC-RN 1200 over the airinterface on a communication link 1400.

The MTC-RN 1200 comprises a transmitter/receiver part 1220 (transmitterand receiver) which transmits and receives data over an air interfaceusing an antenna 1225. The MTC-RN 1200 also comprises a processor 1240which controls the transmitter/receiver part 1220 in receiving andtransmitting data over the air interface. The processor 1240 may alsocontrol the measurement of a received signal strength and propagationdelay from the MTC-UE over the air interface, and is responsible forprocessing UE MAC PDUs into relayed MAC PDUs in the manner describedabove. The MTC-RN 1200 communicates in both the downlink and uplinkdirection with the DeNB 1100 over the air interface on a communicationlink 1400, and receives uplink transmissions from the MTC-UE 1300 overthe air interface on a communications link 1500. The communications link1500 is unidirectional (uplink only), because the MTC-RN 1200 does nottransmit to the MTC-UE 1300.

The MTC-UE 1300 comprises a transmitter/receiver part 1320 (transmitterand receiver) which transmits and receives data over an air interfaceusing an antenna 1325. The MTC-UE 1300 also comprises a processor 1340which controls the transmitter/receiver part 1320 in receiving andtransmitting data over the air interface. The processor 1340 may alsocontrol other functions related to the purpose of the MTC-UE 1300. Forexample, if the MTC-UE is a measurement device intended to report backmeasurements (e.g. temperature, or power usage) via the network, thenthe processor 1340 may be responsible to handling the taking andprocessing of these measurements, as well as for packaging themeasurement data (into MAC PDUs) for transmission. The MTC-UE 1300receives transmissions from the DeNB 1100 in the downlink direction on acommunications link 1600 (solid arrow), and may transmit directly to theDeNB 1100 in the uplink direction on the communications link 1600(dashed arrow) should it be within uplink transmission range. If theMTC-UE 1300 is out of range, then all uplink transmissions from theMTC-UE 1300 to the DeNB 1100 will be made via the MTC-RN 1200.

It will be appreciated that, while the present embodiment describes asingle-hop relay situation, a multi-hop relay architecture could besimilarly envisaged, with the DeNB controlling the scheduling of uplinkresources at each link in a chain of for exampleMTC-UE→MTC-RN1→MTC-RN2→DeNB.

The above description is based on MTC devices operating within an LTEnetwork. It will however be understood that the principles of thepresent invention are not limited to this, and may be applied to othernetworks and other classes of terminal device.

The invention claimed is:
 1. A wireless communications system,comprising: base station; relay device; and terminal device; wherein theterminal device is operable to transmit a message to the base stationvia the relay device; and the relay device is operable to add a relayheader and Medium Access Control (MAC) control elements to the messagereceived from the terminal device, the relay header comprising controlinformation derived by the relay device based on the message receivedfrom the terminal device, the control information controlling subsequenttransmissions of messages from the terminal device to the relay deviceto synchronize a receive time of the subsequent transmissions at therelay device with a receive time of transmissions from other terminaldevices, the relay device further operable to relay the message havingthe added relay header and the added MAC control elements to the basestation, wherein the message is a MAC Protocol Data Unit (PDU), whereinthe relay device does not communicate with the terminal device on adown-link.
 2. A wireless communications system according to claim 1,wherein the base station is operable to transmit the control informationcontained in the relay header to the terminal device; and the terminaldevice is operable to transmit one or more subsequent messages to therelay device based on the control information included in the relayheader.
 3. A wireless communications system according to claim 1,wherein the relay device is operable to measure a received signal powerof the message received from the terminal device; and to set powercontrol information in the control information for controlling a powerlevel of a subsequent transmission from the terminal device to the relaydevice.
 4. A wireless communications system according to claim 3,wherein the power control information specifies a power offset from thereceived signal power.
 5. A wireless communications system according toclaim 3, wherein the terminal device is responsive to the power controlinformation to control the transmission power of a subsequenttransmission to the relay device.
 6. A wireless communications systemaccording to claim 1, wherein the relay device is operable to measure apropagation time delay between the terminal device and the relay deviceof the message transmitted from the terminal device to the relay device;to determine, based on the propagation time delay, a required timingadvance offset for a subsequent transmission from the terminal device tothe relay device; and to specify said timing advance offset in thecontrol information.
 7. A wireless communications system according toclaim 6, wherein the terminal device is responsive to the timing advanceinformation to control the timing of a subsequent transmission from theterminal device to the relay device.
 8. A wireless communications systemaccording to claim 1, wherein the relay device is operable to transmitthe relay header to the base station in relation to an expected messagefrom the terminal device even if the expected message was not correctlyreceived at the relay device.
 9. A wireless communications systemaccording to claim 8, wherein the message from the terminal device isexpected if uplink resources have been allocated to the terminal deviceby the base station.
 10. A wireless communications system according toclaim 1, wherein the relay device is operable to combine a plurality ofmessages received from one or more terminal devices into a relaymessage, the relay header being added to the relay message and having arespective relay control element for each of the plurality of messagesreceived at the relay device, each relay control element comprising thecontrol information for controlling the transmission of subsequentmessages from the terminal device originating the message to which therelay control element relates; and to transmit the relay message to thebase station.
 11. A wireless communications system according to claim 1,wherein the relay header comprises an identification of the terminaldevice which transmitted the message.
 12. A wireless communicationssystem according to claim 10, wherein an expected message from the oneor more terminal devices includes the relay control element in the relayheader even if the expected message is not correctly received at therelay device.
 13. A wireless communications system according to claim12, wherein the relay control element comprises an indication of whetherthe associated message is present in the relay message.
 14. A wirelesscommunications system according to claim 12, wherein the base station isresponsive to the absence of an expected message in the relay message totransmit a negative acknowledgement message to the terminal device. 15.A wireless communications system according to claim 10, wherein therelay header comprises further control information for controlling thetransmission of data between the relay device and the base station. 16.A wireless communications system according to claim 10, wherein therelay control elements in the relay header are in the same order as themessages to which the relay control elements correspond.
 17. A wirelesscommunications system according to claim 15, wherein the relay controlelements in the relay header are positioned after the further controlinformation.
 18. A wireless communications system according to claim 10,wherein when a plurality of messages from the same terminal device arepresent in the relay message, the plurality of messages are positionedwithin the relay message in the order in which they were received at therelay device.
 19. A wireless communications system according to claim10, wherein the relay device is operable to process and relay themessage received from the terminal device at a Medium Access Control(MAC) layer in a protocol stack.
 20. A wireless communications systemaccording to claim 10, wherein the relay message is a MAC Protocol DataUnit; and the base station is operable to process the MAC Protocol DataUnit of the relay message at the MAC layer in a protocol stack toextract a MAC Protocol Data Unit of the message from the terminaldevice; to process the MAC Protocol Data Unit of the message from theterminal device at the MAC layer; and to pass the processed MAC ProtocolData Unit of the message from the terminal device to a higher level inthe protocol stack.
 21. A method of wirelessly communicating data from aterminal device to a base station via a relay device, comprising:transmitting a Medium Access Control (MAC) Protocol Data Unit (PDU)message from the terminal device to the base station via the relaydevice; deriving control information for controlling a transmission ofsubsequent messages from the terminal device to the relay device basedon the MAC PDU message received from the terminal device; at the relaydevice, adding a relay header and MAC control elements to the MAC PDUmessage received from the terminal device to form a MAC relayed PDU, therelay header comprising the control information; and relaying the MACrelayed PDU message to the base station, wherein the relay device doesnot communicate with the terminal device on a down-link.
 22. A basestation for receiving wireless communications from a terminal device viaa relay device, the base station comprising: a receiver for receiving aMedium Access Control (MAC) Protocol Data Unit (PDU) message transmittedfrom the terminal device via the relay device; wherein the MAC PDUmessage comprises a relay header and MAC control elements added by therelay device, the relay header comprising control information, derivedby the relay device based on the MAC PDU message received from theterminal device, for controlling a transmission of subsequent messagesfrom the terminal device to the relay device, wherein the relay devicedoes not communicate with the terminal device on a down-link.
 23. Amethod of receiving wireless communications from a terminal device via arelay device, comprising: receiving a Medium Access Control (MAC)relayed Protocol Data Unit (PDU) message transmitted from the terminaldevice via the relay device; wherein the MAC relayed PDU messagecomprises a relay header and MAC control elements added by the relaydevice to a received MAC PDU message from the terminal device, the relayheader comprising a new logical channel identifier for the MAC controlelements and control information derived by the relay device based onthe received MAC PDU message, the control information controllingsubsequent transmissions of messages from the terminal device to therelay device to synchronize a receive time of the subsequenttransmissions at the relay device with a receive time of transmissionsfrom other terminal devices, wherein the relay device does notcommunicate with the terminal device on a down-link.